Editors’ Vox

Coastal Ocean Warming Adds to CO2 Burden

With coastal oceans around the world changing from the effects of urbanization, rising carbon dioxide levels, and climate warming, recent work begins to find new land-sea linkages.

Coastal salt marsh at Salter Path, North Carolina. Such tidal wetlands are a common feature along the south Atlantic coast of the United States. The warming of marshland and estuaries is releasing carbon dioxide into the coastal ocean, where CO2 levels are going up faster than can be accounted for by atmospheric rise. Credit: Travis (CC BY-NC 2.0)

Those with long memories will recall the debate in the mid-1980s over establishing the first ocean carbon dioxide (CO2) time series stations in the mid-ocean gyres offshore Hawaii and Bermuda. There was then considerable skepticism as to whether useful signals would emerge. How times have changed! The impeccable records now available reveal with startling clarity the rapid changes in ocean pH and CO2, and multiple associated phenomena.

Recent work by Reimer et al. [2017] provides a break-through, although not without difficulty, and indicates what we may expect around the world as both CO2 and temperature simultaneously rise at the highly populated land-sea interfaceBut equivalent observations in the noisier, complicated coastal ocean have proved far more difficult to achieve. Recent work by Reimer et al. [2017] provides a break-through, although not without difficulty, and indicates what we may expect around the world as both CO2 and temperature simultaneously rise at the highly populated land-sea interface. By executing, and analyzing, data from 26 years of cruises and 9½ years of data from a mooring covering the US South Atlantic Bight, they were able to uncover not only the predictable rise in CO2 and reduction in pH, but also strong hints of an added effect from the release of CO2 by warming of estuaries and continental shelves.

It matters little as to whether the background ocean CO2 signal is normally a source or a sink for the atmosphere: by steadily increasing the atmospheric source we will inevitably drive the ocean properties to a higher CO2, more acidic, state. It is also inevitable that as temperatures rise microbes will oxidize organic carbon faster; this is the well-known reason why we buy refrigerators. The characteristics of marine organic matter are overall not that different from that on land; for example, rotting seaweed and coastal muds can smell bad on a hot day.

The time series observations in the central ocean are in water thousands of meters deep and far removed from this effect. But the coastal oceans and estuaries worldwide are surely feeling the impact – and the data in the study begin to reveal this. We can add to this the likely effects from dredging, agriculture, and urbanization. The result then will be highly variable depending on the intensity of these developments and proximity to source. But the results here show that it may be possible to begin to disentangle these processes. Reimer et al. [2017] conclude that “a substantial pCO2 increase across the across the marginal South Atlantic Bight is due to both rising CO2 and increasing temperature on the middle and outer shelves” but that changes in the coastal zone are driven also by oxidation of organic matter from land.

What of the rest of the world? There are vigorous coastal ocean science programs led by strong teams throughout Europe, Asia, and Australia who are also tackling such problems in regions as diverse as coral reefs and in proximity to megacities. One hopes that they too do not need 26 years of data to make useful conclusions.

The fate of organic matter being transported to shelf sediments might be changing too, and adding measurably to the growing ocean CO2 story.It seems likely that by pairing changing O2 and CO2 signals some added constraints may be possible. But there is a warning call here. The continental shelves occupy some 7.6% of the oceanic area – about the size of the entire North American continent. There is much debate over the fate of organic matter in soils. The results of this research hint that the fate of organic matter being transported to shelf sediments might be changing too, and adding measurably to the growing ocean CO2 story.

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